The present invention relates to a rainwater-treating apparatus in a combined sewer system into which wastewater and storm water are combined as a sewer in a confluent flow. The invention also relates to a method for back washing said apparatus. The invention further relates to a sewer-treating system which uses the sewer-treating apparatus in raining (hereinafter referred to as xe2x80x9crainwater-treating apparatus). In the present application, the term xe2x80x9crainwaterxe2x80x9d is used to mean the sewer which is the combined flow of the storm water and the wastewater during raining, therefore xe2x80x9crainwaterxe2x80x9d as a common meaning is discriminated from xe2x80x9cstorm waterxe2x80x9d.
In the combined sewer system for the wastewater and storm water, the flow amount largely varies between non-rainy weather and rainfall weather. In the combined flow type sewer-treating plant, the maximum inflow water amount per hour into the sewer-treating plant during raining is controlled to three times 3Q as much as a design water amount Q that is the maximum inflow water amount per hour when there is no storm water. An ordinary treatment can be effected in the same manner as in non-rainy weather if the water amount does not exceed Q. However, if it is more than Q and not more than 3Q, this exceeds the ordinary treating power of the reaction vessel in the treating plant, and therefore a simple treatment is effected in which the rainwater is subjected to settlement treatment in the first sedimentation basin and then discharged. Further, if the water amount increases and exceeds 3Q, the stormwater is actually discharged directly to a river, etc. without being treated at all.
Therefore, when the water amount increases over Q, almost all pollution substances are discharged to the river, etc. as they are. This results in pollution of the environment. In particular, since the combined sewer systems are employed particularly in many big cities in which the sewer systems have been propagated in early days, the above environmental problems have become manifesting with the development of waterfronts.
To cope with such problems, high-speed rainwater filter apparatuses having layers of floating filter media have been now being positively developed. However, since the thickness of the filter media in the high-speed rainwater filter apparatuses now under investigation is not less than 2 m, and filter media outflow-preventing screens are necessary at lower portions thereof. Alternatively, the apparatus is equipped with a flocculant-feeding mechanism to increase the treating efficiency. In actual application, since a civil equipment, such as an existing first sedimentation basin, having a water depth of 2.5 to 4 m cannot be used, the above filter apparatus is a technique that is required to be newly constructed from a civil structure, resulting in a huge cost required.
The present invention solves the above problems of the prior art, and has been made to provide a rainwater-treating apparatus in the combined sewer system which apparatus can remove a majority of the pollution substances even if rainwater exceeding Q enters or much rainwater exceeding 3Q enter. Further, the present invention provides a rainwater-treating apparatus which can be installed inexpensively by easily modifying an existing first sedimentation basin as well as a method for back washing the apparatus. Other object of the present invention is to provide the rainwater-treating apparatus in the combined sewer system and the back washing method therefor, enabling the accomplishment of the above object with almost no need of maintenance.
The present invention is made to solve the above problems, and its first aspect is to provide a rainwater-treating apparatus in a combined sewer system, said apparatus comprising a plurality of upward flow type high-speed filter vessels each having a layer of a floating filter medium, a common treated water vessel which is located at upper faces of said high-speed filter vessels and is adapted to collect treated water from the high-speed filter vessels, a used wash water vessel which collects used wash water from said high-speed filter vessels, a rainwater inflow channel for distributing the entering rainwater into upper portions of the high speed filter vessels, and rainwater inflow portions for flowing downwardly the rainwater distributed from the rainwater inflow channel under the respective floating filter media of the filter vessels.
The rainwater-treating apparatus according to the present invention can be constructed by partitioning a first sedimentation basin in the conventional combined flow type sewer-treating plant.
A second aspect of the present invention is to provide a sewer-treating system in a combined flow type sewer-treating line, said system comprising a first sewer-treating section and a second sewer-treating section, said first sewer-treating section comprising a sand-settling basin for receiving sewer, a first sedimentation basin provided downstream of the sand-settling basin and a reacting vessel provided downstream of the sedimentation basin, said second sewer-treating section being provided downstream of the sand-settling basin and comprising a plurality of upward flow type high-speed filter vessels each having a layer of a floating filter medium, a common treated water vessel which is located at upper faces of said high-speed filter vessels and is adapted to collect treated water from the high-speed filter vessels, a used wash water vessel which collects used wash water from said high-speed filter vessels, a rainwater inflow channel for distributing the entering rainwater into upper portions of the high speed filter vessels, and rainwater inflow portions for flowing downwardly the rainwater distributed from the rainwater inflow channel under the respective floating filter media of the filter vessels, wherein when an amount of sewer to be treated is not more than a preset design water amount, the sewer is treated in the first sewer-treating section, whereas when the amount of the sewer to be treated is more than the preset design water amount, the sewer treatment is effected in the first sewer-treating section and in addition a portion of the sewer exceeding the preset design water amount is treated in the second sewer-treating section.
The rainwater-treating apparatus can be installed by modifying the existing first sedimentation basin having a height of 2.5-4 m, for example. The reason is as follows. That is, finer filter medium particles are used as compared with the conventional apparatus, the thickness of the floating filter medium layer can be made as thin as around 0.5 to 1 m that is less than 2 m, no lower screen is required, and that an equipment for adding a flocculant is necessary. Consequently, the compact high-speed filter apparatus can be accomplished, and a plurality of the high-speed filter vessels are provided to reduce an amount of wash water to be used at one time, thereby the thickness of the common treated water vessel can be made thin. Further, the rainwater inflow channel preferably distributes the rainwater as a spontaneous falling flow into the rainwater inflow portions of the respective high-speed filter vessels via inflow weirs of the channel so that variations in the water amount may be coped with.
According to a method for back washing the rainwater-treating apparatus and the sewer system in the combined sewer system, back washing is effected with respect to any high-speed filter vessel in the rainwater-treating apparatus which filter vessel increased its preset pressure loss by flowing downwardly the treated water in the common treated water vessel through the high-speed filtering vessel.
As mentioned above, according to the present invention, the upwardly flowing type high-speed filter vessels are provided through partitioning the first sedimentation basin in the combined flow type sewer-treating plant, and a large amount of the rainwater flowing in during raining is distributed into the vessels through the rainwater inflow channel, so that the rainwater can be filtered at a high speed with the charged floating filter medium. Therefore, even if the rainwater enters in an amount of more than Q or if a large amount of the rainwater enters in an amount exceeding 3Q, a majority of the pollution substances can be removed. Furthermore, since the high-speed filter vessels and so forth are provided through partitioning a part of the existing first sedimentation basins, a space for constructing and installing a new civil equipment is not required to be ensured. As explained below in more detail, the apparatus can be operated in the present invention with almost no maintenance.